1. Field of the Invention
This invention relates to an alignment device, and, more particularly, it is concerned with an alignment device suitable for a step-and-repeat type printing device which transfers patterns of integrated circuits, etc. drawn on glass substrates onto silicon wafers.
2. Description of the Prior Art
Year by year, patterns of large-scaled integrated circuits (LSI) tend to become more miniaturized. As one of the lithographic techniques in answer to demands for such miniaturization, attention has been paid to an optical scale-reduction and projection technique. There have so far been promulgated various devices for performing the pattern printing based on this technique (the so-called "scale-reduction and optical printing device"). In these devices, a reticle pattern of a size several times (e.g. ten times) as large as that of a pattern (chip pattern) to be actually printed is projected in a reduced scale onto a silicon wafer (hereinafter simply called "wafer") by an image projecting lens, and printed. In this case, a region that can be printed on the wafer in a single exposure is no more than 14 mm or so in diameter. In order therefore to print the pattern on the entire surface of the wafer, there has been adopted the so-called step-and-repeat system, wherein the wafer mounted on a printing stage is shifted for a certain definite distance, and the exposure operations were repeated a plurality of times.
Also, in the production of the LSI circuit elements, patterns in a plurality of layers are printed in superposition. In this case, unless errors in superposition among the layers are kept below a predetermined value, the function of the resulting LSI element would inevitably be impaired. For instance, for a pattern having the minimum line width of 1 micro-meter, the permissible error in superposition should be 0.2 micro-meters or so at the maximum. Of course, the less this error is, the better. For successively overlaying such plurality of pattern layers, a device for aligning or registering a pattern to be subsequently printed on that pattern which has already been printed on the wafer, i.e., an alignment device, is necessary. For the sake of good understanding, therefore, there will be given hereinbelow explanations on a conventional device which carries out such exposure and printing.
FIG. 1 of the accompanying drawings is a perspective view showing a schematic construction of the conventional device. A reticle R1, on which a pattern has been drawn in the central portion, is positioned above a projection lens L1. An exposure light source (not shown) is positioned further above the reticle R1. The reticle R1 is provided with two transparent parts 30, 31 on its peripheral portion. Each of the transparent parts has a mark 30x, 31y, respectively. Below the projection lens L1, there is disposed a stage S1 which is capable of moving two-dimensionally. A rotational table 32 is so provided in the stage S1 that it may slightly rotatable relative to the stage S1. (The table 32 is deleted in FIGS. 2, 3 and 4 to avoid complication of the drawing.) A reticle-x microscope RX1 and a reticle-y microscope RY1 have, on the real image planes, reference marks for defining the center of observation, respectively, and are so disposed that the center of observation of each microscope coincides with each axis of the orthogonally intersecting coordinate axis system x-y, as shown in FIG. 1, and detect marks 30x, 31y on the reticle R1, respectively, so as to place the reticle R1 at its predetermined position. Here, it is assumed that the coordinate axes X, Y denote the position of the stage S1 with respect stationary coordinate system. It is also assumed that the axis X and the axis Y of the X-Y coordinate system substantially coincide with the center axes of two laser beams from laser interferometers, 33, 34 and that the intersection of the axis X and the axis Y is adjusted so as to substantially coincide with the optical center of the projection lens L1.
Mirrors 5, 6 fixed to the stage S1 are so arranged as to orthogonally intersect each other. The surfaces of the mirrors 5, 6 are perpendicular to the axes X and Y, namely the laser beams of the laser interferometers 33 and 34.
Relative positioning of the coordinate system x-y and the coordinate system X-Y is done by use of a cross mark C1 fixed on the stage S1. More specifically, when the stage S1 is so moved that the image of the cross mark C1 through the projection lens L1 coincides with the center of observation of the microscope RX1, the position of the stage S1 relative to axis X is measured by the interferometer 33. The relative positions of the axis Y and the axis y can also be determined in the same manner.
By the side of the projection lens L1, there are disposed microscopes WXY and W.theta.1, each having an optical axis which is substantially parallel with the optical axis of the projection lens L1.
The microscopes WXY, W.theta.1 are provided with reference marks "+" and "-" on the real image plane thereof, respectively, as shown in FIG. 2. The reference marks "+" "-" serve as the observation centers of the microscopes WXY, W.theta.1.
When a wafer W1 is set on the rotational table 32 of the stage S1 in FIG. 2, the microscope WXY is used to determine the position of the wafer with respect to the axes X and Y of the coordinate system X-Y, and the microscope W.theta.1 is used to determine the rotational position of the wafer. The cross mark C1 is brought to the center of observation of the microscope WXY by moving the stage S1. At this instant, when the values of coordinates of the stage S1 in the coordinate system X-Y are measured by the interferometers 33, 34, the position of the center of observation of the microscope WXY can be found. Subsequently, the stage S1 is moved only in the direction X of the coordinate system X-Y to view the cross mark C1 by the microscope W.theta.1. Then, the center of observation of the microscope W.theta.1 in the direction Y is moved so that it may coincide with the line 1 in the cross mark C1 extending in the direction X. In this manner, adjustment for coinciding the center of observation in the direction Y of both microscopes WXY and W.theta.1 is carried out. In this adjusted condition, the wafer is mounted on the rotational table 32 of the stage S1 and the positioning, i.e., alignment, of the wafer will be performed.
FIG. 2 is a top plan view of the device shown in FIG. 1, wherein the wafer W1 is mounted on the stage S1. It should be understood that an X-Y alignment mark 3 (hereinafter simply referred to as "XY-mark 3") and a .theta. alignment mark 4 (hereinafter simply referred to as ".theta.-mark 4") are provided in advance at positions on the wafer W1 corresponding to the center of observation of the microscopes WXY, W.theta.1, respectively. The stage S1 is then shifted from its state shown in FIG. 2 to the state shown in FIG. 3. Subsequently, the wafer W1 is rotated by rotation of the rotational table in such a manner that the microscopes WXY, W.theta.1 become able to view the XY-mark 3 and the .theta.-mark 4 at their respective centers of observation. When the XY-mark 3 and the .theta.-mark 4 are exactly positioned by the wafer rotation, the coordinate position of the stage S1 is measured by the interferometers 33, 34, and the value is memorized. By finding the difference between this memorized coordinate position and the coordinate position of the center of observation of the microscope WXY, the position of the wafer W1 with respect to the coordinate system X-Y is determined. In this way, the position of the wafer W1 can be determined accurately with respect to the XY-mark 3 and the .theta.-mark 4. However, should there be accompanied yawing at the time of movement of the stage S1, errors in the positioning would inevitably occur. This will be explained briefly in the following.
Generally speaking, since this type of stage is required for high speed movement, there exists a yawing of approximately 0.5 second in terms of an angular movement, even with the most precisely manufactured stage. Further, depending on the stage, the yawing takes place when the direction of movement of the stage changes, and the quantity of yawing varies depending on the speed of the stage movement. Furthermore, the nature of the yawing generally differs from stage to stage.
As an example, it is assumed that a distance LY1 from the axis X of the coordinate system X-Y to the microscope WXY is 70 mm. It is further assumed that yawing of 0.5 second would occur when the stage S1 is so moved that the cross mark C1 varies from its position just above the axis X to its position right beneath the microscope WXY. Under these conditions, the Abbe error becomes approximately 0.17 micro-meter (70.times.10.sup.-3 .times.0.5/60.sup.2 .times..pi./180). Accordingly, the alignment standard of the wafer W1 in the direction X deviates by a quantity equal to this error with respect to the coordinate system X-Y, whereby the wafer W1 which is aligned in accordance with this standard would have the same quantity of error (alignment error) in the direction X. The microscopes WXY, W.theta.1 are disposed at their respective positions away from the axis Y of the coordinate system X-Y. In consequence of this, there would also take place the same deviation in alignment, due to the same Abbe error as mentioned above, in the direction Y, i.e., the rotational direction, of the wafer W1.
As explained in the foregoing, the alignment error in the wafer constitutes a problem when the patterns in a plurality of layers are to be printed in superposition by one and the same printing device. This problem also takes place when the patterns in a plurality of different layers are to be printed by different printing devices, which hinders interchangeability of the printing device.
In the following, an explanation will be given with reference to FIG. 4 as to why the microscopes WXY, W.function.1 are disposed at their respective positions away from the axes X, Y.
FIG. 4 shows a similar layout as in FIG. 3. In general, the projection lens L1 is required to have an effective diameter of 14 mm for its projecting plane so that a square pattern region of about 10 mm square may be printed on the wafer W1. Further, within the circle of this effective diameter, the projection lens L1 should be able to project a pattern on the wafer W1 under the telecentric condition and print thereon the same having a line width as thin as approximately 1 micro-meter. Therefore, the projection lens L1 should eventually have a diameter D which is greater than a predetermined value. Actually, the value of the diameter D is 100 mm or so, and it is difficult to be made smaller than this. A diameter d of the microscopes WXY, W.theta.1 should be at least 20 mm or so. At the time of the wafer W1 alignment, the spacing between the microscopes WXY and W.theta.1 is preferably as long as possible for accurate measurement of the rotational error of the wafer W1. That is, if the size of the wafer W1, on which a pattern is to be printed, is taken to be three inches (approx. 76.2 mm), the spacing (2.multidot.LX1) between the microscopes WXY and W.theta.1 should be made as close to three inches as possible. Therefore, when the interval 2.multidot.LX1 is set to approximately 70 mm, the microscopes WXY and W.theta.1 would deviate from the Abbe condition by LX1.apprxeq.35 mm. Furthermore (D+d)/2.apprxeq.60 mm or longer would be required for a distance LY1 from the axis X to the center of observation of the microscope WXY. In reality, the distance LY1.apprxeq.70 mm or so is the minimum length in consideration of parts to hold the microscopes WXY and W.theta.1. Accordingly, they are deviated from the Abbe condition by the amount of distance LY1.